1. Technical Field
The invention relates to an electric machine with stator teeth of irregular geometry for accommodating electrical windings.
2. Background Art
A stator for an electric machine capable of acting as an electric motor and as an electric generator typically has an iron core comprising a stack of laminations surrounding a rotor with an air-gap therebetween. The rotor also has an iron core. In the case of a permanent magnet electric machine, the rotor has sets of permanent magnets that are inserted in openings in the rotor core. The stator laminations have radial slots in which the stator windings are located. The windings carry an electrical excitation current to produce torque. The portions of the laminations between two consecutive slots may be referred to as stator teeth, which channel magnetic flux. In a conventional multiple phase machine, the teeth are of identical shape and size.
Separate groups of stator teeth and the stator windings form multiple magnetic poles that produce a flux flow pattern when the stator coils are energized with a multi-phase sinusoidal voltage. A three-phase electric machine, for example, would have a total of 8 poles and 48 slots. A group of 6 slots would be characteristic of each pole of the particular examples of a 48 slot electric machine herein disclosed. The magnetic flux created by the stator windings interacts with rotor flux created by a rotor for the electric machine so that a rotor torque is created as the stator windings are excited with a multi-phase voltage.
As explained in co-pending patent application Ser. No. 12/566,723, filed Sep. 25, 2009 published Mar. 31, 2011 as U.S. 2011-0074243, entitled “Stator for an Electric Machine”, which is assigned to the assignee of the present invention, the torque created by the interaction of the flux field created by the stator windings and the flux field created by the rotor develops a uniform torque component and a varying torque component. The varying torque component is developed by the stator and the rotor because of harmonic magnetic flux distributions in the air-gap. The total output torque of the electric machine is a combination of both components. Because of the variable torque component, a torque ripple phenomenon is created, which results in motor torque output speed oscillations when the electric machine is acting as a motor. If the electric machine is used to create torque in an electric vehicle powertrain, the torque ripple may cause driveline speed oscillations, which can result in vehicle vibration and noise as electric machine vibrations resonate in the vehicle body and chassis structure.
The stator flux and rotor flux are distributed through an air-gap that exists between the outer periphery of the rotor and the inner periphery of the stator. The largest component of the two flux distributions is called the fundamental component. During normal operation of the electric machine, the stator and the rotor fundamental flux rotate in the same direction and at the same speed. Thus, the interaction between the stator and the rotor fundamental fluxes generate a constant torque. Because of the slot openings in the stator, the air-gap permeance is not a constant, which causes so-called harmonic fluxes whose pole number, rotation speed and direction are different from the pole number, rotation speed and direction of the stator and rotor fundamental fluxes. The interaction between the different harmonic fluxes generate variable torques associated with torque ripple. The torque ripple has different components that have different frequencies. The order of a torque ripple is defined as a ratio of the frequency of the torque ripple to the rotary speed of the rotor.
For most applications, the torque ripple needs to be reduced to a manageable level, particularly in the case of hybrid electric vehicle powertrain applications where the torque ripple components occur at variable frequencies proportional to the output shaft speed of an electric traction motor. Higher order of frequencies usually can be filtered out by a limited bandwidth in the mechanical components of the powertrain. Lower frequencies, however, give rise to mechanical oscillations that cannot readily be filtered. Such oscillations are not acceptable in a hybrid electric vehicle powertrain. Presence of torque ripples from the motor at the lower frequencies that cause the motor to produce undesirable vibration and noise.
In application Ser. No. 12/268,592, filed Nov. 11, 2008 entitled “Permanent Magnet Machine with Offset Pole Spacing”, the shape of the torque ripple is affected by adjusting the geometry of permanent magnets carried by the rotor. By using magnets in an asymmetric distribution pattern, the total electric machine torque ripple can be reduced in magnitude. The '592 application is assigned to the assignee of the present invention.
A further known method for reducing torque ripple involves a rotor construction consisting of multiple sections that are skewed in small angular increments, one section with respect to the other, so that a magnetic pole of one section is angularly disposed with respect to the pole axis of an adjacent rotor section.